Because of their substantially globular molecular architectures, hyperbranched polymers are envisioned to possess interesting properties such as low viscosity and high thermal stability and serve as functional materials. Realization of the full potential of hyperbranched polymers calls for the exploration of new, versatile methods for their syntheses. A variety of hyperbranched polymers have been prepared by condensation polymerization of so-called ABn-type (n≧2) monomers wherein A and B functions condense together to form branched polymers, for instance, hyperbranched polyester and polyamide polymers prepared by Frechet, J. M. J. (U.S. Pat. No. 5,514,764). Competitive reactions, however, often lead to the formation of linear chains, producing imperfect polymers with a low degree of branching and low molecular weight. Cyclotrimerization of alkynes is a reaction in which three alkynes cyclize to form a benzene ring. It was first reported by Berthelot, M. in 1866 (Ann. Chim. Phys., 1866, 9, 445) and has now been developed into a chemo-, regio-, and stereoselective reaction for the synthesis of organic molecules for theoretical and practical interests. Cycloaddiation of diacetylenes (or diynes) has been used as an elementary reaction for the construction of hyperbranched polymers with high molecular weight and degree of branching because the polycyclotrimerization mechanism is intolerant of the formation of linear repeat units inside the hyperbranched core. For instance, Olsen, R. E. prepared a novel polyphenylene resin by copolycyclotrimerization of vinylethynylbenzene, diethynylbenzene, and phenylacetylene (U.S. Pat. Nos. 5,686,027 and 5,594,058) initiated by Ziegler catalysts. The obtained polymer is used in the construction of high-performance carbon-carbon composites. However, the reaction system is complex. Ben Zhong Tang's group has successfully synthesized soluble hyperbranched poly(alkenephenylene)s and poly(arylene)s by diyne polycyclotrimerization with tantalum and cobalt complexes as the catalysts (Macromolecules, 2004, 37, 5196 and C. R. Chimie, 2003, 6, 833).
The drawbacks of the method mentioned above are that the polymers produced are regioirregular in structures (random mixtures of 1,3,5- and 1,2,4-trisubstituted benzenes) and the transition-metal catalysts are intolerant to functional groups. To overcome the problems, the present invention utilizes polycyclotrimerizations of diaroylacetylenes to prepare regioselective 1,3,5-triaroylbenzene-based hyperbranched (co)polymers. Cyclotrimerization of benzoylacetylenes was first reported by Balasubramanian, K. (Synthesis, 1980, 29). The reaction is known to proceed in a strictly regioselective way due to the involved ionic mechanism (J. Org. Chem. 2002, 67, 4547). Moreover, the reaction is carried out without using transition-metal catalysts but only in refluxing DMF (dimethylformamide) or using secondary amines as catalysts. It thus may be utilized to prepare hyperbranched polymers from diaroylacetylenes bearing functional groups and cut the production costs tremendously.
The only known example of branched polymer containing triaroylbenzene moieties was reported by Tiwari, R. K. in 1996 (Indian Journal of Chemistry, 1996, 35B, 1263), which was prepared by polycyclotrimerization of terephthaloylacetylene. He aimed to synthesize trans-1,4-bis(3-aryloxy-2-propenoyl)benzenes from base-catalysed addition of phenols to terephthaloylacetylene. However, the reaction yielded large amount of insoluble cyclotrimerized polytribenzoylbenzene. Since the polymer is not processible in most of the organic solvents, it is hard to characterize its structure and find any commercial applications.
Benzophenone has been introduced into natural (e.g., protein) and synthetic polymers (e.g., polyimide) to serve as a photocross-linker. For instance, a preimidized benzophenone photoactive polymer is commercially available as Ciba-Geigy 412. A pattern is then projection exposed on the polymer using a mercury arc lamp at an output of 1200 mJ/cm2 at 365 nm to initiate crosslinking. However, during the development and post-baking, the film swells and shrinks greatly. Thus, patterning of the 412 polymer has been limited to the formation of vias on the order of millimeters in size. Agostino, P. A invented a new method of patterning a preimidized benzophenone photoactive polymer used as a photoresist with a laser light source to increase its crosslinking efficiency and density, thereby resulting in reduced swelling during development (U.S. Pat. No. 5,370,974). But, the resolution is still micronmeter and the resist film must be further heated beyond the flow point to high enough temperatures to initiate widespread thermal crosslinking in the bulk of the resist.